A lithium secondary battery is a secondary battery having a structure, in which, at the time of charging, lithium begins to dissolve as ions from a positive electrode and moves to a negative electrode to be stored therein, while, conversely, at the time of discharging, the lithium ions return from the negative electrode to the positive electrode. Since the lithium secondary battery has high energy density and a long life cycle, it is widely used as a power supply for home appliances such as a video camera, portable electronic equipment such as a notebook computer and a portable telephone, and electrically-drive tools such as a power tool. Recently, the lithium secondary battery is also applied to a large-sized battery that is mounted in an electric vehicle (EV), a hybrid electric vehicle (HEV), or the like.
This kind of a lithium secondary battery is constituted of a positive electrode, a negative electrode, and an ion conducting layer inserted between both of the electrodes. As the ion conducting layer, a separator constituted of a porous film, such as polyethylene and polypropylene, which is filled with a non-aqueous electrolytic solution, is generally used. However, since such an organic electrolytic solution using a flammable organic solvent as a solvent is used as an electrolyte, it is required to improve the structure and material for preventing volatilization or leakage, and also, it is required to install a safety device for suppressing an increase in temperature at the time of a short circuit and to improve the structure and material for preventing a short circuit.
In contrast, an all-solid lithium secondary battery that is constituted by solidifying the whole battery using a solid electrolyte does not use a flammable organic solvent, and thus, it is possible to attempt the simplification of a safety device. In addition, the battery can be made excellent in the production cost or productivity. Furthermore, it is possible to attempt high voltage by laminating it in a cell in series. Furthermore, for this kind of a solid electrolyte, a Li ion only moves, and thus, there are no side reactions by the movement of anion. Therefore, it is expected that it leads to improve safety or durability.
As this kind of a solid electrolyte, there is disclosed a sulfide-based solid electrolyte including Li7PS6, Li4P2S6, Li3PS4, or the like, in a crystal phase, as a solid electrolyte exhibiting high lithium ion conductivity (For example, see Patent Document 1).
Among them, the crystalline solid electrolyte including Li3PS4 in a crystal phase is chemically stable, and also, has high conductivity. Therefore, it is a material that is particularly brought to attention. In addition, it is considered that an amorphous solid electrolyte is decomposed when it is immersed in an organic solvent, and thus, a solid electrolyte having high crystallinity (refers to as “crystalline solid electrolyte”) is practically preferable.
As for such a crystalline solid electrolyte including Li3PS4 in a crystal phase, for example, Patent Document 2 discloses a sulfide solid electrolyte, which has good ionic conductivity, the composition of Li(4−x)PxGe(1−x)S4 (x satisfies 0<x<1), the peak at the position of 2θ=29.58°±0.50° for X-ray diffraction measurement using a CuKα line, and less than 0.50 of the ratio (IB/IA) of the peak diffraction intensity IB of 2θ=27.33°±0.50° to the peak diffraction intensity IA of 2θ=29.58°±0.50°.
By the way, in the case of forming a solid electrolyte layer using this kind of a solid electrolyte, it is performed that the slurry obtained by mixing, grinding, and then, slurrying a dispersion medium composed of a solid electrolyte and an organic solvent is applied on a substrate to form a coating film for forming a solid electrolyte layer, and the film is dried to form a solid electrolyte layer.
However, the sulfide solid electrolyte including sulfur has very high reactivity, and thus, the dispersion medium, which can be used at the time of preparing slurry, is limited to a non-polar solvent such as toluene or heptane.
In this regard, Patent Document 3 discloses that it is also possible to use a dispersion medium composed of at least one of tertiary amine; ether; thiol; ester having a functional group having 3 or more of carbon atoms, which is bound to the carbon atom of an ester group, and a functional group having 4 or more of carbon atoms, which is bound to the oxygen atom of an ester group; and ester having a benzene ring which is bound to the carbon atom of an ester group, as a solvent (dispersion medium) when slurrying the sulfide solid electrolyte, as a solvent other than a non-polar solvent.